Disc drives read and write information along substantially concentric tracks formed on discs. Each of the concentric tracks is divided into a plurality of sectors. Each of these sectors usually includes a servo field and a data field that are contiguous. To locate a particular track on a disc, disc drives use the embedded servo fields in the sectors. The embedded servo fields are utilized by a servo sub-system to position a head over a particular track on a rotating disc.
In most current disc drives, the servo fields are written onto the disc in-situ (i.e., after the disc is mounted on the spindle motor of a disc drive) when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position during operation. Ideally, a head following the center of a track moves along a perfectly circular path around the disc. However, various types of errors prevent heads from following this ideal path. One type of error is a written-in error that arises during creation of the servo fields. Written-in errors occur because the write head used to produce the servo fields does not always follow a perfectly circular path due to unpredictable pressure effects on the write head from the aerodynamics of its flight over the disc, and from vibrations in the gimbal used to support the head. Because of these written-in errors, a head that perfectly tracks the path followed by the servo write head will not follow a circular path. Written-in errors are often referred to as repeatable run-out (RRO) errors or written-in repeatable run-out (WI-RRO) errors because they cause the same position errors each time the head passes along a track.
Instead of using the above-described discs with in-situ written or post-written tracks, some current disc drives include discs with servo-tracks that are pre-written onto the discs before the discs are mounted on the spindle motor of the drive. When discs with pre-written tracks (pre-written discs) are mounted on a spindle motor of a disc drive, there is a certain amount of misalignment of the track center of the disc and the spin axis of the spindle motor. Because of such disc mounting tolerances, the pre-written tracks are eccentric to the spin axis of the spindle motor. This eccentricity of the tracks causes additional RRO errors.
To address the additional RRO, in certain types of disc drives with pre-written discs, instead of attempting to follow the eccentric tracks during normal drive operation, compensation signals are injected into a servo control loop of the servo-subsystem to cancel the RRO and cause the head to follow virtual tracks that are concentric with the axis of rotation of a disc. Virtual track parameters or physical track (written track) misalignment compensation parameters, which are used to produce the compensation signals, are computed and stored in flash memory during manufacture of the disc drives. In such drives, virtual track parameters are usually stored in flash memory because, if these parameters are written to the media on virtual tracks, it would be impossible to seek to these tracks the first time the drive is used. Utilizing additional flash memory to store the virtual track parameters and other manufacturing process data (mapped flawed sectors, offsets between heads in the drive and other data computed during manufacture of the disc drive) is costly and complex.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.